Solenoid valves having permanent magnets as biasing members are known inter alia from U.S. Pat. Nos. 3,202,447, 4,306,589, 4,489,863, 4,561,632, 4,574,841, 4,690,371, 5,318,071, 5,345,968, and 5,351,934.
Conventional solenoid valves comprise three ports, a plunger biased by a spring into a first position, known as the normal position, in which it closes a first port and leaves a fluid path open through the remaining two ports. A solenoid surrounds the plunger, and the plunger essentially forms the core of a magnetic circuit comprising the plunger, the solenoid and further connecting parts of ferrous material, a gap being provided between the plunger and the entrance to one of the two open ports. When current is supplied to the solenoid the plunger moves against the bias of the spring to close the magnetic gap, and thus the first port, previously closed, is opened and one of the other ports is closed, to redirect the fluid path from the newly closed port to the newly opened port. This is the powered position and the valve can only remain in this state as long as the solenoid continues to be powered, otherwise it is returned to the normal position by the bias in the spring. In particular U.S. Pat. No. 5,727,769 discloses a valve which consists of a solenoid which drives a plunger in a yoke to open and close a valve part. A permanent magnet is peripherally positioned around the yoke in a predetermined position such that the magnetic field drives the plunger. The permanent magnet holds the plunger by the position of a groove part in the plunger. The magnetic field forms a loop facing the axial direction of plunger movement with a very short gap.
An advantage of this device is that the system restrains vibration of the plunger by making use of the eddy currents that are generated in proportion to the speed of the plunger. The device is able to achieve a rapid braking of the plunger at a predetermined position. This reduces impact damage on the plunger and thus improves the lifetime of the device.
A disadvantage with many of the devices mentioned above is the amount of energy usage. The solenoid coils are fairly thin and easily heated. A device of this kind may contain or be located very near plastic. In addition heating effects mean that the duty cycle of the device has to be kept below a certain maximum. Furthermore there are many applications in which it is preferable to keep electricity consumption to a minimum, for example in automatic irrigation equipment where the device may be left to operate for several hours a day over a large land area for the duration of the growing season. There are other application where it is essential to keep electricity consumption to a minimum, for example when the valve is located on portable equipment where the only power supply is a battery. Even valves on vehicles that include an alternator to generate electricity from the engine may be required to operate at times when the engine is not operational. For example valves that are part of a system for providing hot water to de-ice the outside of the vehicle are required to operate before the vehicle starts moving. Not only is their electric supply restricted to the vehicle battery but most of the available supply is needed to heat the water.
U.S. Pat. No. 5,727,769 discloses a solenoid valve in which a permanent magnet is placed outside the solenoid in order to damp the oscillation of the coil and thereby to enhance controllablity of the coil.
U.S. Pat. No. 4,295,769 discloses a solenoid valve in which a permanent magnet serves as a latch and in which a detaching coil is provided to generate a magnetic field which partially temporarily nullifies the magnetic field.